Neutron counter filled with boron trifluoride gas

ABSTRACT

Failure of neutron detecting chambers can be avoided if the results of fault analysis are properly analyzed. In this invention the analysis leads to a redisposition, redesign and reorientation of ceramic components to avoid or delay their deterioration during use of the chamber.

United States Patent Goodings et al.

[451 Nov. 7, 1972 [54] NEUTRON COUNTER FILLED WITH BORON TRIFLUORIDE GAS[72] Inventors: Anthony Goodings, Winfrith; John Walgate Leake, Harwell,both of England [73] Assignee: United Kingdom Atomic Energy Authority,London, England [22] Filed: June 29, 1970 [21] App]. No.2 50,762

[30] Foreign Application Priority Data July 1, 1969 Great Britain..33,290/69 [52] US. Cl ..313/61 D, 313/93, 313/242, 313/281 [51] Int.Cl .1101] 39/32 [58] Field of Search .....313/61 R, 61 D, 93, 281, 242

[56] References Cited UNITED STATES PATENTS Cade ..313/242 X 2,505,9195/1950 Simpson, Jr. ..313/93 2,605,435 7/1952 Krasnow et al ..313/932,835,839 5/1958 Borzin ..313/93 2,845,560 7/1958 Curtis et al. ..313/93X 2,879,423 3/1959 Bayard ..313/93 Primary Examiner-Roy Lake AssistantExaminerPalmer C. Demeo Attorney-Larson, Taylor & Hinds [57] 7 ABSTRACTFailure of neutron detecting chambers can be avoided if the results offault analysis are properly analyzed. In this invention the analysisleads to a redisposition, redesign and reorientation of ceramiccomponents to avoid or delay their deterioration during use of thechamber.

5 Claims, 1 Drawing Figure NEUTRON COUNTER FILLED WITI-I BORONTRIFLUORIDE GAS BACKGROUND OF THE INVENTION This invention relates toneutron counters for detecting and measuring neutron flux (i.e.neutrons/cm /sec).

An important method of detecting neutrons and measuring neutron flux isthat of employing the B (naz)Li reaction and as a matter of practicalitythis involves constructing an ionization chamber which is either coatedinternally with a boron containing compound or which contains a countervolume charged with BF3 gas. As constructed hitherto these detectorshave a somewhat limited operational life and it is an object of thepresent invention to provide an improved design.

An important cause of failure is the transfer by migration or depositionof alien metal on to ceramic insulation exposed to the chamber interior.The presence of this metal has a deleterious effect on the electricalinsulation and/or sealing properties of the ceramic which leads tofailure of the chamber. The source of the metal may be either themetallic braze material which forms a gas seal between the ceramic andmetal parts of the chamber or it may arise from the boron content of thechamber, for example boron may plate on to the ceramic surfaces from thechamber exposed to the filling gas.

SUMMARY OF THE INVENTION According to the invention a neutron sensitivedetector is provided which comprises a metal body defining a chamber, ananode extending within the chamber in spaced relation to the chamberwall, a ceramic to metal seal through which electrical signals from theanode are lead by a conductor insulated from the body, the ceramic tometal seal having a ceramic portion which protrudes into the chamber todefine an extended surface of ceramic material. Preferably theprotruding ceramic portion is shielded from the main chamber volume by ashroud.

One effect of the extended surface is topostpone the effect whichmigrating braze material has in bridging the ceramic surface between theparts insulated by the ceramic; it also provides a greater surface areafor possible metal plate out so that the deposition is not on to aconcentrated sensitive area.

DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENT Asshown, the counter comprises a 1 inch diameter aluminum tube 1 closed ateach end by aluminum plugs 2, 3. The closed volume of the tube ischarged with boron trifluoride gas and a coaxial tungsten anode wire issupported centrally on the tube axis as will be more fully explainedbelow.

The plug 2 has a central aperture bounded by a reentrant portion to theinner end face to which is friction welded a stainless steel sleeve 4.The bore of the sleeve 4 is of stepped diameter and in the smaller boreportion is brazed a metal tube 5 having an internal shoulder. Brazed tothe shoulder bore is a ceramic to metal seal feature designatedgenerally at 6 whose function is to form a gas-tight, electricallyinsulating seal between the wire 10 and the end plug 2 in the casing land permit the necessary ionizing potential between those parts to bemaintained. Electrical insulation is provided by a ceramic sleeve 6awhich is fixed in a short metal sleeve 6b by a copper-silver braze whichalso provides a gastight seal between these parts. The metal sleeve 6bis itself brazed into the bore of the internal shoulder of the tube 5.

It will be observed that the forward part 7 of the ceramic sleeve 6aextends forwardly into the cylinder volume for no obvious reason. Thisextension 7 is in fact provided so as to form an extended surfaceforming a longer path for any migrating material from the copper-silverbraze so that such material would take longer to bridge the insulation.Moreover the extended surface provides additional area for thedeposition of boron from the chamber atmosphere. Nevertheless, theextension 7 does not extend beyond the terminal part of the tube 5 whichin practice shrouds the extended surface area from the main volume ofboron containing gas within the tube.

At its rear end face (external of the chamber) the ceramic sleeve 6aabuts and engages and seals against the end of a tube 8, the junctionbetween 6a and 8 being enclosed by a perforated cap 9. The tube 8supports coaxially within it a long thin walled tube 11 which extendsthrough the plug 2 on the cylinder axis protruding both backwards andforwards of the seal feature 6. At its rear end, the tube 11 is brazedto the wire 10 which is the central anode, collector, electrode of thecounter volume. The front end of the tube 11 defines one limit of theactive volume of the chamber.

The other end of the active volume is defined by a further tube 12through which the wire 10 passes. The wire end is here fused into aglass bead 13 which itself is fused on to the tube 12. The tube 12 is asliding fit in a central hole in a silica disc 14 perforated with holes15 and fixed peripherally to the cylinder wall. The disc provides oneabutment for a spring 13a which bears on the head 13 and correctlytensions the wire 10. The disc 14 has a number of concentric grooves 14acut in its faces to increase the electrical length of the surface andhence deminish the effect of substances which reduce the insulatingproperties of the disc, such as boron.

The end plug 3 is in peripheral sealing engagement with the tube 1.There is a central aperture in it however which receives the fillingstem 16. The stem 16 is an aluminum tube through which BF3 gas at theappropriate pressure is introduced into the cylinder. After filling, thestem 16 is crimped and sealed by a sealing weld 17. Finally an end cap18 is tack welded in place to prevent accidental damage to the stem 16.

A boron counter as described above has been operated and found toexhibit enhanced resistance to the effects of boron plating out from thefilling gas on to the internal surfaces of the chamber.

We claim:

l. A neutron sensitive detector comprising a metal body defining acylindrical gas tight chamber, a volume of BF3 gas contained within saidchamber, the chamber having closed ends, a ceramic to metal seal closingone of said ends, an elongated electrode extending coaxially within thechamber in spaced relation to the wall, the electrode being supported atone'of its ends in the ceramic to metal seal which electricallyinsulates the electrode from the metal body, the ceramic to metal sealhaving a ceramic portion extending into the chamber, and a metal shieldshrouding the extended ceramic portion from the atmosphere within thechamber 2. A neutron sensitive detector as claimed in claim 1 in whichthe elongated electrode is supported within the chamber at its endremote from the ceramic to metal seal by an annular ceramic memberextending from the chamber wall, the member having grooves therein, eachsaid groove having a width greater than 4 its depth.

3. A neutron sensitive detector as claimed in claim 2 in which theelectrode is supported in the ceramic member by a tensioning devicewhich holds the electrode in tension.

4. A neutron sensitive detector as claimed in claim 1 in which theelectrode is supported in the ceramic to metal sealby a metal alloybraze, the extended surface of the ceramic extending from the metalbraze into the chamber, and the metal shield shrouding that extendedsurface.

5. A neutron sensitive detector as claimed in claim 1 comprising analuminum tube defining said metal body, a stainless steel sleeve, afriction welded connection joining said sleeve within said tube at oneend thereof, the stainless steel sleeve supporting the ceramic to metalseal which in turn supports the electrode.

2. A neutron sensitive detector as claimed in claim 1 in Which theelongated electrode is supported within the chamber at its end remotefrom the ceramic to metal seal by an annular ceramic member extendingfrom the chamber wall, the member having grooves therein, each saidgroove having a width greater than its depth.
 3. A neutron sensitivedetector as claimed in claim 2 in which the electrode is supported inthe ceramic member by a tensioning device which holds the electrode intension.
 4. A neutron sensitive detector as claimed in claim 1 in whichthe electrode is supported in the ceramic to metal seal by a metal alloybraze, the extended surface of the ceramic extending from the metalbraze into the chamber, and the metal shield shrouding that extendedsurface.
 5. A neutron sensitive detector as claimed in claim 1comprising an aluminum tube defining said metal body, a stainless steelsleeve, a friction welded connection joining said sleeve within saidtube at one end thereof, the stainless steel sleeve supporting theceramic to metal seal which in turn supports the electrode.